1. Field of the Invention
The present invention relates to an image recording apparatus capable of controlling image recording conditions according to the density of the original image.
2. Description of the Prior Art
There are already known image recording apparatus, such as copiers, which measure the background density of an original image in response to a detection signal from density detecting means for detecting the image density of the original image and accordingly controls certain image recording conditions such as the amount of image exposure or the image developing condition, thereby automatically obtaining an appropriate density in the reproduced image.
As examples of such an image recording control device for controlling the image recording conditions through the detection of the original image density, Japanese Patent Laid-open No. 42856/1975 and Japanese Patent Laid-open No. 2134/1979 disclose a system in which the original image density is directly detected by an optical sensor positioned close to the optical system for such control. In such known system, the original image density or the background density is measured by reading the integrated value in the scanning of the original image with the optical system, or by reading the minimum value in the detection signal from said optical sensor.
There is also known a system in which the image recording conditions are controlled by detecting, with a potential sensor, the surface potential of a photosensitive member, which corresponds to the original image density, and the background density of the original image is determined, in the similar manner as in the aforementioned case of density detection with the optical sensor, by reading the integrated value or the minimum value in the scanning direction.
However, such conventional image recording control device may not be able to properly control the image recording conditions for certain original images since, as mentioned above, the background density of the original image is determined by reading the integrated value or the minimum value, in the scanning direction, of the detection signal from the density detecting means for detecting the original image density.
For example, the background density determined by the integrated value of the density detection signal in the scanning direction is close enough to the actual background density in case the original image contains a large proportion of the background or contains characters and fine lines as the image, as shown in FIG. 1A. However, if the original image contains a wide black image area as shown in FIG. 1B, the background density determined from the integrated value becomes significantly different from the actual background density. This is due to the limit in the resolving power of the optical or potential sensor employed for detecting the original image density, as characters and fine lines (0.1-0.5 mm in width) cannot be resolved by such sensor while a broad line exceeding a certain width and a certain density level can be resolved. More specifically, such sensor with an insufficient resolving power senses characters and fine lines only as a part of the background, but the background density thus determined approximately coincides with the actual background density since in most originals the characters and fine lines only represent about 10 per cent of the entire area. However, a broad line (exceeding 5 mm in width) as shown in FIG. 1B can be easily detected with a sensor of a low resolving power, so that the background density determined from the integrated value of the sensor signal in the scanning direction always becomes higher than the actual background density because the obtained integrated value contains such broad line as a part of the background. The image recording control in such state tends to obtain an appropriate contrast by compensating the apparently elevated background density. More specifically, as the sensor reads a so-called background smudge of the original, there is conducted a compensating process, for example by increasing the exposure to the photosensitive member, whereby the reproduced image becomes generally thinner.
Also the background density determined from the minimum value of the density detection signal may become significantly different from the actual background density in certain original images, and is therefore unable to provide the reproduced image of an appropriate density.